This invention relates to Common Channel Signaling System 7 and, in particular, to Message Transfer Part level 3 signaling to an independent signaling controller device.
The Signaling System No. 7 (SS7) protocol has been mandated for out-of-band signaling communication involving telecommunication network elements and has become a defacto worldwide standard which has been adopted by the International Telecommunication Union (ITU), American National Standards Institute (ANSI), Telcordia Technologies and European Telecommunications Standards Institute (ETSI). SS7 networks and protocols are used for efficient and secure worldwide telecommunications by standardizing data and telephone call setup, management, and tear down. SS7 messages are exchanged between network entities in dedicated bi-directional channels called signaling links. Signaling occurs out-of-band on the signaling links rather than in-band on voice channels or circuits. Furthermore, the out-of-band signaling provides for faster call setup times and more efficient use of voice circuits than in-band signaling.
There are three general types of network entities that are commonly found in SS7 networks. FIG. 1 is an illustration of a conventional SS7 communication network. A Service Switching Point (SSP) network entity 102, FIG. 1, is a class five telephonic type switch that originates, terminates, or passes telephone/data calls from a telephonic device 104. The SSP 102 sends signaling messages to other SSPs 106 to setup, manage, and release voice circuits 110 located at the other SSPs 106 as required to complete telephone/data calls.
A Service Control Point (SCP) network entity 114 is a centralized database used to determine how calls are routed (e.g. 1-800 and 1-888 numbers in North America). The SCP 114 sends a response to the originating SSP 102 containing the routing numbers associated with a dialed number. A call routing feature may allow an alternate routing number to be used by the SSP 102 if the primary dialed number is busy or the call is unanswered within a specified time. Actual call features implemented on the SCP 114 do vary from network to network and from feature to feature.
A Signal Transfer Point (STP) network entity 116 is a signaling switch for routing incoming messages on a signaling link 118 to an outgoing signaling link 118 based on the routing information contained in the SS7 message. The STP 116 acts as a hub in the SS7 signaling network improving utilization of the network by eliminating the need for direct signaling links between SSPs. Additionally, the STP 116 may perform global title translation, a procedure by which the destination network entity is determined from digits present in the signaling message.
SS7 signaling protocol is composed of layered protocols and may be mapped to the Open System Interconnection (OSI) seven layer reference model. A SS7 protocol stack is shown in FIG. 2. The first SS7 layer is the Message Transfer Part (MTP) level 1 202, FIG. 2, and is equivalent to OSI Physical Layer. The MTP level 1 202 defines the physical electrical, and functional characteristics of a signaling link. Examples of the physical interfaces defined by MTP level 1 202 include E-1 (2048 kb/s; 32 64 kb/s channels), DS-1 (1544 kb/s; 24 64 kb/s channels), V.35 (56 kb/s), DS-0 (64 kb/s), and DS-0A (56 kb/s).
MTP Level 2 protocol 204 is the messaging that ensures transmission of a message between two network elements. MTP level 2 protocol 204 implements flow control, message sequence validation, error checking, and is equivalent to the OSI Data Link Layer. When an error occurs on a signaling link, a message is retransmitted. MTP level 2 protocol 204 defines three kinds of messages or signaling units.
The first message is the Fill-In Signal Units (FISUs) which are transmitted continuously on a signaling link in both directions unless other signal units are present. The FISUs carry basic level 2 protocol information only, such as an acknowledgment of signal unit receipt by a remote signaling point. The second message is the Link Status Signal Units (LSSUs) which carries one or two octets of link status information between signaling points at either end of a link. the signaling link status is used to control signaling link alignment and indicates the status of a network entity to another network entity. The third message is the Message Signal Units (MSUs) carrying all call control, database query and response, network management, and network maintenance data. MSUs have a routing label which allows an originating signaling point to send information to a destination signaling point across the network.
MTP level 3 protocol 206 ensures accurate end-to-end transmission of a message across a network and provides message routing between network elements in the SS7 network and is equivalent in function to the OSI Network Layer. MTP level 3 protocol 206 routes messages based on the routing label in the signaling information field (SIF) of the message signal units. The routing label is comprised of the destination point code (DPC), originating point code (OPC), and signaling link selection (SLS) field. Point codes are numeric addresses which normally uniquely identify each network entity in the SS7 network and may be used as the address which identifies the user parts at that entity.
The DPC identifies a SSP 102, FIG. 1, with the switching fabric controller integrated together with the signaling controller having the same destination point code. All messages for trunk management must be routed to the integrated signaling controller of the SSP 102. Once the messages are received at the signaling controller, the data contained in the messages is processed and the required actions are given to the integrated switch fabric controller (the switch fabric being the collection of voice or data links between SSPs).
The MTP level 3 protocol 206, FIG. 2, contains the MTP level 3 message routing label which identifies the originating and destination SS7 devices. Network devices, such as STP 116, FIG. 1, have message transfer capabilities that enables call and/or data traffic to be re-routed away from failed links and signaling points in addition to controlling traffic when congestion occurs.
Level 4 protocol 208 and higher in the SS7 signaling protocol are for control of the voice channels and considered application level protocols. Some examples of application level protocols are the Telephone User Part (TUP) and Integrated Service User Part (ISUP). The SCCP protocol 210 is a MTP User Part that provides connection-less and connection-oriented network services to protocols above MTP level 3 protocol 206 that are not related to the voice channels. While the MTP level 3 protocol 206 provides routing labels that enable messages to be addressed to specific network entities, the SCCP protocol provides subsystem numbers that allow messages to be addressed to specific applications running on an individual network entity. The SCCP protocol 210 is used as the transport layer for services such as 1-800/888 service, calling card, wireless roaming, and personal communication services (PCS).
The SCCP protocol 210 provides the means by which a STP 116, FIG. 1, can perform global title translation (GTT), a procedure by which the destination network entity and subsystem number (SSN) is determined from digits present in the signaling message. The global title digits, for example, may be the dialed 800/888 number, calling card number, or mobile subscriber identification number depending on the service requested. Because a STP 116 provides global title translation, originating network entities do not need to maintain a database of destination point codes and subsystem numbers associated with specific services and possible destinations. Therefore, SCCP GTT can be used to identify routes to specific network elements.
The redirecting of the messages at the SCCP protocol level 210, FIG. 2, to another network entity results in the message being sent to a network device such as SCP 114 which contains the database for processing the routing digits. Information is processed from the SCCP 210 messages, such as the dialed digits, and used as an index into a database. The index into the database results in a MTP level 3 206 destination point code being identified for the SCCP message.
In turn, SCCP 114 used the SCCP 210 to send a reply back to the originating element which has an integrated switch fabric, such as a SSP (PSTN telephonic switch). A disadvantage of GTT is the functionality resides at the MTP user level and requires additional processing of messages resulting in increased message processing at transfer points. In a network, redundant STP 116 elements that have the GTT functionality may be paired and both assigned an additional common signaling point code to be used only so that MTP level 3 206 can address their common point code and have redundant access to the function. The SCCP aliasing is limited by the SCCP protocol messages being directed to the GTT processing elements. Accordingly there is along felt need in the art to permit general aliasing of point codes at MTP level 3, especially to extend the concept to the ISDN User Part, resulting in more reliable processing of messages in the SS7 network.
The problems noted above are solved in accordance with the invention and a technical advance is achieved in the art, by using a Service Switching Point that has the SS7 signal controller independent from the switching fabric network element. The MTP level 3 messages containing alias destination point codes results in messages being routed to network devices with minimal processing and increased reliability of the Service Switching Points. Additionally, by allowing the separation of the switching fabric from the MTP level 3 messaging interface the aliasing at the MTP level 3 protocol solves the problem of only being able to send to integrated switch fabrics and signaling controllers by extending the aliasing capability to multiple Call Controllers.